20 Mangrove Ecosystems Diversity of Mangrove Fauna It might be expected that faunal species diversity would follow a similar pattern to that of mangrove tree diversity both because the mangrove fauna has presumably been exposed to the same influences and because of a presumption that faunal diversity should respond to tree diversity The IWP region, richer in plant diversity than the ACEP, is also richer in species of mangrove-associated Crustacea and mollusks (Table 3) The reverse is true of other taxonomic groups, particularly those that form constituents of the root communities, such as sponges, coelenterates, and echinoderms This may reflect regional differences in tidal range and availability of roots for settlement For many groups, unfortunately, little comparable data are available and recorded species numbers reflect taxonomic interest and effort rather than the composition of actual species assemblages Regional Patterns of Diversity Species diversity varies within regions in response to many different factors The ACEP region, in addition to having fewer mangrove species in total, shows less differentiation between localities within the region, and all the species available in the geographical vicinity are likely to be represented at most locations Various factors may result in local variation in species diversity Mangroves not grow on rocky shores or in areas where fresh water is completely lacking (which is in part why all tropical shores are not dominated by mangroves) Stretches of inhospitable coastline therefore act as barriers which affect mangrove dispersal and geographical distribution The arid shores of Somalia, for example, result in the reduction in species number northwards so that Avicennia marina is virtually the only mangrove species found in the Red Sea Separation of mangrove estuaries from each other by arid coastline, and regional-scale variation in physical variables, also affects the species distribution of mangroves around the Australian coasts Dispersal ability also affects species distributions within regions The number of mangrove species on islands of the western Pacific shows clear attenuation with increasing distance from the species-rich areas of Australia and Papua New Table Guinea Similarly, among islands off the West African coast there is a clear relationship between the number of mangrove species present and the distance from the nearest landward neighbor (Figure 12) Species number also correlates with island size, with larger islands containing more species Local Variation in Species Distribution and Diversity Tree Distribution At a specific location, the distribution of mangrove species responds to physical variables in the environment These often vary as gradients: in an estuarine mangal, for instance, salinity and the influence of tidal fluctuations tend to diminish with distance up the river Sediment composition and nutrient dynamics also alter with distance from the open sea Mangrove species respond differentially to such upriver/downriver gradients, resulting in zonation of species Similarly, in areas dominated by tide rather than river flow, tidal fluctuations establish gradients of physical variables, particularly in salinity and the extent of waterlogging of the soil Again, mangrove species respond differentially to these physical variables and tend to form distinct zones Where both river and tidal influences interact, the pattern of species distribution can be extremely complex In relation to salinity, species generally grow better at low salinity and differ more in the tolerance range than in their salinity optima Low salinity, in consequence, tends to be associated with higher species diversity At higher salinities, tolerance differences result in differing competitive success and translate into zonation of mangrove species along a salinity gradient, with species dominating zones at which they compete best, rather than those corresponding to salinity growth optima Although response to physical gradients suggests a gradual transition from one species to another as the determining physical variable gradually alters, this is often not the case Mangrove species are frequently found in virtually monospecific stands or zones, with a more or less abrupt transition from one dominant species to another This suggests that interactions between tree species, and mutual exclusion, may play a part in defining zone boundaries Other physical Number of species recorded from mangroves in various localities in the regions indicated Taxonomic group Sponges/bryozoa Coelenterata/Ctenophora Nonpolychaete worms Polychaetes Crustacea Mollusks Echinoderms Ascidians Fish Reptiles Birds Mammals Atlantic–Caribbean–East Pacific Indo-West Pacific Caribbean/Western Atlantic East Africa Indo-Malesia Australia 36 42 13 33 87 124 29 30 212 138 12 72 163 117 23 13 114 13 11 229 211 74 35 128 145 10 156 244 283 22 177 36